Multi-step processes for high temperature bonding and bonded substrates formed therefrom

ABSTRACT

A method for high temperature bonding of substrates may include providing a top substrate and a bottom substrate, and positioning an insert between the substrates to form a assembly. The insert may be shaped to hold at least an amount of Sn having a low melting temperature and a gap shaped to hold at least a plurality of metal particles having a high melting temperature greater than the low melting temperature. The assembly may be heated to below the low melting temperature and held for a first period of time. The assembly may further be heated to approximately the low melting temperature and held for a period of time at a temperature equal to or greater than the low melting temperature such that the amount of Sn and the amount of metal particles form one or more intermetallic bonds. The assembly may be cooled to create a bonded assembly.

TECHNICAL FIELD

The present specification generally relates to methods and apparatusesfor high temperature bonding and substrates formed therefrom and, morespecifically, methods and apparatuses for high temperature bondingapplying multi-step heating processes to at least a pair of substratesto form a strengthened bond layer.

BACKGROUND

Power semiconductor devices, such as those fabricated from SiC (siliconcarbide), may be designed to operate at very high operating temperatures(e.g., greater than 250° C.). Such power semiconductor devices may bebonded to a cooling device, such as a heat sink or a liquid coolingassembly, for example. The cooling device removes heat from the powersemiconductor device to ensure that it operates at a temperature that isbelow its maximum operating temperature. The bonding layer that bondsthe power semiconductor device to the cooling device must be able towithstand the high operating temperatures of the power semiconductordevice.

Transient liquid phase (TLP) or diffusion bonding or soldering aremethods of high temperature bonding that may be applied. For example,TLP bonding results in a bond layer having a high temperature meltingpoint. A typical TLP bond consists of two different material compounds:a metallic layer and an intermetallic layer or alloy. Generally, theintermetallic layer is formed during an initial melting phase wherein alow melting temperature material, such as tin, diffuses into highmelting temperature materials, such as copper or nickel. Although theintermetallic alloy has a high re-melting temperature, conventionalprocesses use tin along with paste including metal particles such ascopper or nickel and apply a direct, one-step heating. The assembly isheated to a temperature that is, for example, a low melting point oftin. However, with such one-step heating, the paste impedes the path ofthe formed tin solder and prevents the solder from more fully coatingother metal particles, which may result in a weaker bond layer.

Accordingly, a need exists for alternative methods for high temperaturebonding of substrates for forming a strengthened bonding layer between apair of substrates.

SUMMARY

In one embodiment, a method for high temperature bonding of substratesincludes providing a top substrate and a bottom substrate andpositioning an insert between the top and bottom substrates to form aassembly. The insert is positioned around at least a portion of the topsubstrate and includes at least one of a porous material and one or morechannels. The insert includes a gap shaped to be disposed between thetop and bottom substrates and that is shaped to hold at least an amountof a plurality of metal particles having a high melting temperature. Atleast one of the insert and the gap is configured to hold at least anamount of tin (Sn) having a low melting temperature, and the highmelting temperature is greater than the low melting temperature. Themethod includes heating the assembly during a first heating to a firsttemperature that is below the low melting temperature, holding theassembly at the first temperature for a first period of time, andheating the assembly during a second heating to a second temperaturethat is approximately equal to the low melting temperature. The methodfurther includes holding the assembly at a holding temperature for asecond period of time such that the amount of Sn and the amount of metalparticles form one or more intermetallic bonds, wherein the holdingtemperature is equal to or greater than the second temperature, andcooling the assembly to create a bonded assembly.

In another embodiment, a bonding assembly includes a bonding assemblyincluding a top substrate, a bottom substrate, and an insert. The topsubstrate includes one of SiC or silicon (Si). The bottom substrateincludes one of direct bonded copper substrate or direct bonded aluminumsubstrate. The insert includes at least one of a porous material and oneor more channels. The insert is disposable between the top substrate andthe bottom substrate to form a bond layer and disposable around at leasta portion of the top substrate. The insert includes a gap disposedbetween two adjacent surfaces of the top and bottom substrates. Theinsert is utilized to hold at least an amount of Sn having a low meltingtemperature and an amount of a plurality of metal particles having ahigh melting temperature. The high melting temperature is greater thanthe low melting temperature.

These and additional features provided by the embodiments describedherein will be more fully understood in view of the following detaileddescription, in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments set forth in the drawings are illustrative and exemplaryin nature and not intended to limit the subject matter defined by theclaims. The following detailed description of the illustrativeembodiments can be understood when read in conjunction with thefollowing drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1A schematically depicts a first bonding assembly prior to forminga bond layer between a pair of substrates per the method of FIG. 1,according to one or more embodiments shown and described herein;

FIG. 1B schematically depicts the first bonding assembly of FIG. 2Aafter undergoing the method of FIG. 1, in which the first bondingassembly has a formed bond layer between the pair of substrates,according to one or more embodiments shown and described herein;

FIG. 2 schematically depicts a top plan view of an example insert of thefirst bonding assembly of FIG. 1A, according to one or more embodimentsshown and described herein;

FIG. 3 schematically depicts a top plan view of an alternative exampleinsert of the first bonding assembly of FIG. 1A, according to one ormore embodiments shown and described herein;

FIG. 4A schematically depicts another first bonding assembly prior toforming a bond layer between a pair of substrates per the method of FIG.1, according to one or more embodiments shown and described herein;

FIG. 4B schematically depicts the another first bonding assembly of FIG.4A after undergoing the method of FIG. 1, in which the another firstbonding assembly has a formed bond layer between the pair of substrates,according to one or more embodiments shown and described herein;

FIG. 5A schematically depicts another first bonding assembly prior toforming a bond layer between a pair of substrates per the method of FIG.1, according to one or more embodiments shown and described herein;

FIG. 5B schematically depicts the first bonding assembly of FIG. 5Aafter undergoing the method of FIG. 1, in which the yet another firstbonding assembly has a formed bond layer between the pair of substrates,according to one or more embodiments shown and described herein;

FIGS. 6A-6E schematically depict another first bonding assembly prior toforming a bond layer between a pair of substrates, according to one ormore embodiments shown and described herein;

FIG. 6F schematically depicts the first bonding assembly of FIGS. 6A-6Eafter undergoing the alternative method, in which the first bondingassembly has a formed bond layer between the pair of substrates,according to one or more embodiments shown and described herein;

FIGS. 7A-7F schematically depict first bonding assembly prior to forminga bond layer between a pair of substrates, according to one or moreembodiments shown and described herein;

FIG. 7G schematically depicts the first bonding assembly of FIGS. 7A-7Fafter undergoing the alternative method, in which the first bondingassembly has a formed bond layer between the pair of substrates,according to one or more embodiments shown and described herein;

FIG. 8 depicts a flow chart of a multi-step method for high temperaturebonding of substrates, according to one or more embodiments shown anddescribed herein;

FIG. 9 depicts a flow chart of another multi-step method for hightemperature bonding of substrates, according to one or more embodimentsshown and described herein;

FIG. 10 depicts a flow chart of another multi-step method for hightemperature bonding of substrates, according to one or more embodimentsshown and described herein;

FIG. 11 graphically depicts a temperature profile employable for one ormore embodiments described and illustrated herein.

DETAILED DESCRIPTION

Referring generally to the figures, embodiments of the presentdisclosure are directed to methods and apparatuses for high temperaturebonding of substrates. The methods include providing a pair ofsubstrates and positioning an insert holding tin (Sn) and a plurality ofmetal particles in a gap therebetween to form a assembly. The insert maybe porous and/or may include one or more channels. The Sn has a lowmelting temperature, and the plurality of metal particles have a greaterhigh melting temperature. Further, paste may be positioned between thetwo substrates. The paste may include an organic binder that includesthe metal particles. A first heating step brings the assembly to atemperature below the low melting temperature and is held at thistemperature for a period of time. During the first heating step, pastethat was present between the two substrates may be evaporated orvaporized. Any evaporated or vaporized paste may be exhausted from theassembly through, for example, the porous pathways or channels of theinsert. The methods further include a second heating step to atemperature approximately equal to the low melting temperature and asecond holding step. During the second heating step, tin is drawn intothe forming bond layer between the substrates via a capillary force towet the metal particles and is consumed into the bond layer. During thesecond holding step at a temperature equal to or greater than the lowmelting temperature, one or more intermetallic bonds may be formed.Because the paste was vaporized during the first heating step, theformation of intermetallic bonds is not impeded by residual paste. Themethods include cooling the assembly to create a bonded assembly havinga bond layer that bonds the substrates.

As stated above, processes described herein provide for the advantagesof creating a stronger bond layer over a conventional one-step heatingmethod for high temperature bonding of substrates. The evaporation ofpaste during the first heating step creates a less impeded path (whichcauses a better wetting of particles than a more impeded path wouldpermit) for the formation of intermetallic bonds during the secondheating step because it provides for a better wetting of particles. Thisless impeded path may provide for a stronger bond layer over one thatwould be formed in a case where the paste was not fully evaporated orvaporized. Such an at least two-step heating process as described hereinmay additionally provide for the strengthening of the bond layer inapplications where paste is not utilized to contain the metal particlessuch as in instances where a powder including metal particles isutilized. For example, the metal particles may be provided in a powder.

Various embodiments of methods for high temperature bonding ofsubstrates and substrates formed therefrom are described in detailherein. Although example methods for high temperature bonding ofsubstrates are described in the context of power electronicsapplications (e.g., to bond a power semiconductor device to a coolingassembly in an inverter circuit of hybrid or electric vehicles), the useof methods described herein is not limited thereto. For example, examplemethods and substrates formed therefrom that are described herein may beimplemented in other semiconductor use applications and otherapplications to bond two components together.

FIG. 1A illustrates a schematic of an example bonding assemblyassembled, and FIG. 8 illustrates an example method for high temperaturebonding of substrates utilizing, for example, the bonding assembly ofFIG. 1A. Referring to FIGS. 1A and 8, a top substrate 200 and a bottomsubstrate 202 is provided as set forth within block 800 of FIG. 8. Thetop substrate 200 includes a die that is made of Si (silicon), SiC(silicon carbide), or the like. The bottom substrate 202 includes directbonded copper, direct bonded aluminum, or the like.

As set forth in block 802 of FIG. 8, and as shown in FIG. 1A, an insert204A is positioned between the top substrate 200 and the bottomsubstrate 202 to form a assembly 105A. The insert 204A is positionedaround at least a portion the top substrate 200. For example, FIG. 1Ashows the insert 204A positioned around a lower portion of an exteriorperimeter surface of the top substrate 200. The insert 204A is made of amaterial capable of withstanding the melting temperatures describedherein, such as a graphite, stainless steel, or a like suitablematerial. In some embodiments, the insert 204A may be made of a porousmaterial, such as, without limitation, porous copper. The insert 204Amay be circular or another shape that is configured to match the portionof the top substrate 200 that it surrounds. In some embodiments, theinsert 204A may include one or more channels to allow for a pasteevaporation pathway, as described in more detail below. The channels maybe provided by the porous structure, or by dedicated channels within theinsert.

As a non-limiting example, the insert 204A may include a thickness inthe range of about 1 mm to 2 mm. The insert 204A defines a gap G that isshaped to be disposed between the top substrate 200 and the bottomsubstrate 202. In the illustrated embodiment, the gap G is shaped tohold at least an amount of a plurality of metal particles, such as metalparticles 208 shown in FIG. 1A. The gap G also holds at least an amountof tin (Sn), such as tin 206 shown in FIG. 1A. Additionally oralternatively, the insert 204A may be made of a porous material that mayhold tin 206.

As shown in FIG. 1A, the insert 204A may include a lip 212 that inwardlyextends from an exterior portion of the insert 204A (the exteriorportion having an exterior surface S) and includes walls that form thegap G. The lip 212 is disposed below the exterior surface S to form alip protrusion portion within the interior of the insert 204A. FIGS. 2and 3 schematically illustrate a top-down view of the example inserts204, 204′ (e.g., insert 204A of FIG. 1A or insert 204C of FIG. 5A asdescribed further below). For example, as shown in FIG. 2, the lip 212may include a lip surface L that forms a generally rectangular shapedgap G. As another non-limiting example, and as shown in FIG. 3, the lip212, of the insert 204′, may include a lip surface L that form agenerally oval shaped gap G, and other shapes and configurations arealso possible.

Referring to FIGS. 1A-3, the lip 212 includes lip surfaces L upon whichportions of surfaces of substrates 200 and/or 202 are disposed to rest.Any suitable design or shape for the lip surfaces L of lip 212 thatshapes gap G to a desired shape capable of holding an amount of tin 206and metal particles 208 in a form to creates a bond layer 210 (FIG. 1B)as described herein or through conventional high temperature bondingprocesses are within the scope of this disclosure.

Alternatively, as shown in FIGS. 4A and 4B, the insert 204B may notinclude a lip 212 but rather may be flush with edges of substrates 200and 202. For example, assembly 405A shows a top substrate 200 disposedabove a bottom substrate 202 with an insert 204B disposed about andflush against a top portion of the bottom substrate 202 and a bottomportion of the top substrate 200. The insert 204B includes a top surfaceS that has an end abutting against the bottom portion of the topsubstrate 200 and a bottom surface that abuts against a ledge surface ofthe bottom substrate 202. Similar to the insert 204A, the insert 204Bmay be porous or have one or more channels to act as evaporationpathways for an organic solvent, organic binder, and/or flux asdescribed in greater detail further below. After the insert 204Bundergoes an example heating method for high temperature bonding ofsubstrates, such as the method shown in FIG. 8, the bonded assembly 405Bof FIG. 4B is created as the bond layer 210 between substrates 200, 202is formed.

Additionally or alternatively, as shown in FIGS. 5A and 5B, the insert204C may include a lip 212 having a slot 214 that may retain tin 206(FIG. 5A). For example, assembly 505A shows a top substrate 200 disposedabove a bottom substrate 202 with a portion of an insert 204C disposedabout and flush against a top portion of the bottom substrate 202 and abottom portion of the top substrate 200. The insert 204C includes a topsurface S that has an end abutting against the bottom portion of the topsubstrate 200 and a bottom surface that abuts against a ledge surface ofthe bottom substrate 202. The insert 204C further includes a lip 212having lip surface L and extending into a space between substrates 200,202. The lip 212 includes a slot 214 that is shaped to hold tin 206, forexample. Similar to inserts 204A-204B, the insert 204C may be porous orhave one or more channels to act as evaporation pathways for an organicsolvent, organic binder, and/or flux as described in greater detailfurther below. After the insert 204C undergoes an example heating methodfor high temperature bonding of substrates, such as the method shown inFIG. 8, the bonded assembly 505B of FIG. 5B is created as the bond layer210 between substrates 200, 202 is formed.

The amount of tin 206 and the amount of metal particles 208 may dependon the desired application and properties for the resulting bond layer.As a non-limiting example, the amount of tin 206 may include a weightpercent of 70% Sn, and the amount of the plurality of metal particles208 may include a weight percent of 30%. The plurality of metalparticles may be Ni, Cu, Al, Ag, or like metal materials, such as metalmagnetic materials, alone or in combinations thereof. In embodiments,the material of the metal particles 208 may comprise at least about 30wt % copper, at least about 30 wt % nickel, at least about 30 wt %silver, at least 30 wt % aluminum, and/or a 30 wt % mixture of Ni, Cu,Al, and/or Ag. In other embodiments, the material of the metal particles208 (e.g., Ni, Cu, Al, Ag, other suitable like metal materials, or anycombination thereof) includes a weight percent of in the range of about20% to about 40%, and the tin (Sn) 206 comprises a respective weightpercent in the range of about 80% to about 60%. In embodiments, thematerial of the metal particles 208 may comprise Ni, Cu, Al, and/or Agand be in a range of at least about 20 wt % to at least about 40 wt %.For example, the amount of tin 206 may include a weight percent of 60%Sn, and the amount of the plurality of metal particles 208 may include aweight percent of 40% Ni. Or the amount of tin 206 may include a weightpercent of 60% Sn, and the amount of the plurality of metal particles208 may include a weight percent of 40% Cu. Or the amount of tin 206 mayinclude a weight percent of 80% Sn, and the amount of the plurality ofmetal particles 208 may include a weight percent of 20% Ag.

The tin 206 has a low melting temperature that is lower than the meltingtemperature of the metal particles 208. Tin has a melting point in therange of about 230° C. to about 260° C. Accordingly, the assembly 205may be heated to temperature approximately the low melting temperatureof tin, or rather, be heated to a range of about 250° C. to 300° C. orabout 230° C. to about 260° C. such that the tin within the assembly 205melts. The metal particles have a high melting temperature that isgreater than the low melting temperature of the tin. Thus, during theexample heating processes and methods described below, when the tin 206is heated up to its melting temperature to eventually melt to create asolder that will coat the metal particles 208, the metal particles 208themselves do not melt and create a solder.

Referring once again to FIG. 8, in block 804, the assembly 105A isheated during a first heating to a first temperature that is below thelow melting temperature of tin 206. The first heating, as well as otherheatings described herein, may be part of a transient liquid phase (TLP)heating or soldering or a diffusion soldering process as conventionallyknown.

In block 806, the assembly 105A is held at the first temperature for afirst period of time. As examples and not as limitations, the firsttemperature may be in the range of about 170° C. to about 180° C. Thefirst period of time may be approximately equal to or about 5 minutes.By holding the assembly 105A at the first temperature for a first periodof time, a substantial amount of paste that may be present in theassembly 105A may be evaporated or vaporized.

As stated above, the metal particles 208 are positioned between the topsubstrate 200 and the bottom substrate 202. The plurality of metalparticles 208, for example, may be configured as loose particles in theform of a powder. In such embodiments, the powder may be positioned onsurfaces of the top substrate 200 and the bottom substrate 202 such thatthe powder is disposed between substrates 200 and 202. In otherembodiments, the plurality of metal particles 208 provided in a pasteincluding an organic binder, such that the plurality of metal particles208 is disposed in the organic binder. The organic binder may be anorganic ingredient as understood by those skilled in the art that isused to bind together two or more materials. In a non-limiting example,the paste may be deposited on adjacent surfaces of top substrate 200 andbottom substrate 202. The paste alternatively may include a fluxcomponent, at least one of tin 206 and the plurality of metal particles208, and an organic solvent (i.e., having a viscosity which is less thana viscosity associated with an organic binder).

In embodiments in which the metal particles 208 are provided in a paste,the step of holding of the assembly 105A at the first temperature (e.g.,block 806 of FIG. 1) vaporizes or evaporates the non-metallic particlecontaining portion (such as an organic binder or solvent) of the paste.Thus, substantially no organic binder from the paste will be present toimpede the path of tin solder from tin 206 onto the plurality of metalparticles 208 to form intermetallic bonds after the second heating to asecond temperature (e.g., block 808 of FIG. 8) and the second period oftime holding at the second temperature (e.g., block 810 of FIG. 8), asdescribed in greater detail below.

In block 808, the assembly 105A is heated during a second heating to asecond temperature that is approximately to the low melting temperatureof the tin 206 such that the tin 206 is drawn and consumed into a bondlayer 210 disposed between substrates 200, 202. The drawing action mayoccur via a capillary force such that Sn is drawn inside and consumedinto the bond layer. In embodiments, and referring to FIGS. 1A, 4A, and5A, the tin 206 is drawn in directions of A and B and wets the pluralityof metal particles 208. In block 810, the assembly 105A is held at atleast the second temperature for a second period of time such that theamount of tin 206 and the amount of metal particles 208 formintermetallic bonds. Thus, the assembly is held at a holding temperaturefor the second period of time, where the holding temperature is equal toor greater than the second temperature. For example, if the assembly105A is heated to a second temperature in block 108 that isapproximately the low melting temperature, such as the meltingtemperature of tin, in block 810 the assembly 105A may be held at thesecond temperature for a second period of time such as in the range ofabout 5 minutes to about 10 minutes to allow capillary forces assistwith the action of drawing Sn into an eventual bond layer.

As an example and not a limitation, the second temperature may be in therange of about 230° C. to about 260° C. or in the range of about 250° C.to about 300° C. As another non-limiting example, the second temperaturemay be approximately equal to 230° C. or approximately equal to 250° C.As a non-limiting example, the second period of time may be in the rangeof about 30 minutes to about 60 minutes.

As another example, after the assembly 105A is heated during the secondheating to the second temperature that is approximately equal to the lowmelting temperature of the tin 206, the assembly 105A may continue to beheated to a holding temperature that is a greater than the low meltingtemperature of the tin 206. Thus, in block 810, the assembly 105A isheld at the holding temperature, which is at least the secondtemperature or higher, such that the assembly 105A is held at at leastthe second temperature. If the holding temperature is greater than thesecond temperature, between blocks 808 and 810, the assembly 105A may beheated to the holding temperature in a continuous heating or in aramped-up, incremental heating.

FIG. 9 shows an example of an incremental heating method that will bedescribed in greater detail below with respect to FIGS. 6A-6F. In FIG.9, blocks 900-910 and 920 respectively parallel blocks 800-810 and 812of FIG. 8, and blocks 912-918 of FIG. 9 outline further incrementalheating steps. For example, prior to the cooling step of the block 812of FIG. 8 (or respective block 920 of FIG. 9) as described below, theassembly 105A may be increased to a third temperature (i.e., in block912) that is slightly higher than the second temperature and held for athird period of time (in block 914) to assist to fill gap G with tin206. The third temperature may be, for example, in the range of about250° C. to about 270° C., and the third period of time may be in therange of about 5 minutes to about 10 minutes. Then, the assembly 105Amay be increased to a fourth temperature (in block 916) that is slightlyhigher than the third temperature (and that may be equal to the holdingtemperature). The assembly 105 may then (in block 918) held at thefourth temperature for the second period of time to, for example, form abond layer 210 via a reaction between the amount of tin 206 and theamount of metal particles 208 that form intermetallic bonds. The fourthtemperature may be, for example, in the range of about 290° C. to about300° C., and the second period of time may be in the range of about 30minutes to about 40 minutes or about 30 minutes to about 60 minutes.

FIG. 10 shows an alternative method (that is described in greater detailbelow with respect to FIGS. 7A-7G) in which blocks 1000-1006respectively parallel blocks 800-806 of FIG. 8. However, in block 1008,and after holding an assembly such as the assembly 105A at the firsttemperature, the assembly may be continuously heated up to a secondtemperature that is above the melting point of tin (in block 1010) andheld at the second temperature (in block 1010) prior to being cooled (inblock 1012).

FIG. 11 graphically depicts a heating process in which an examplebonding assembly, as according to one or more of the embodimentsdescribed herein, is heated within the first 10 minutes to a firstholding temperature in the range of about 150° C. to about 200° C. Forexample, the bonding assembly may take about 5 minutes to heat to thefirst holding temperature range, and then the bonding assembly may beheld at the first holding temperature range for about 5 minutes. Then,as set forth in FIG. 10, the bonding assembly may be heated to a secondholding temperature in the range of about 250° C. to about 350° C. andbe held for a period in a range of about 30 minutes to about 120minutes. For example, the bonding assembly may be ramped up to thesecond holding temperature range within a period of from about 5-10minutes. Alternatively, as set forth in FIG. 9, the second holdingtemperature range may be incrementally ramped up to the second holdingtemperature range and held for periodic incremental periods. The bondingassembly may then be cooled (as set forth in FIGS. 8-10) to create abonded assembly.

Referring once again to FIG. 8 and the example assembly 105A of FIG. 1B,in block 812, the assembly 105A is cooled to create a bonded assembly105B as shown in FIGS. 1B. Block 812 may similarly be applied createbonded assembly 405B of FIG. 4B or bonded assembly 505B of FIG. 5B. Thethickness of the bond layer may depend on the application and is notlimited by this disclosure. In embodiments, and referring once again toFIG. 1B as an example, the bond layer 210 of the bonded assembly 105Bhas a thickness in the range of about 10 μm to about 250 μm aftercooling as set forth in block 812. In embodiments, the bond layer 210may have a thickness that is at least about 10 microns (μm), at leastabout 20 microns, at least about 50 microns, at least about 100 microns,at least about 200 microns, or even at least about 250 microns. Inadditional embodiments, the thickness of the bond layer 210 may be lessthan about 250 microns, less than about 200 microns, less than about 100microns, less than about 50 microns, less than about 20 microns, or evenless than about 10 microns.

While the inserts 204A-204C of FIGS. 1A, 4A, and 5A described abovedepict embodiments in which a gap G is shaped to hold both tin 206 andthe metal particles 208, each of which may be held as either a paste orpowder, in alternative embodiments, such as those described below withrespect to FIGS. 6A-7G, one or more walls of an example porous insertmay be shaped to hold tin 206. For example, the gap G of insert 204D ofFIGS. 6A-6F is shaped to hold an organic binder including the metalparticles 208 while the insert 204D initially holds the tin 206. Thus,FIGS. 6A-6F depict an alternative embodiment of a bonded substrateassembly 605F, which is also formed through an alternative method asdescribed in FIG. 9, as described below. It should be understood,however, that other heating methods described herein may be applied toassemblies 605A-605F as shown in respective FIGS. 6A-6F.

FIG. 6A shows an assembly 605A including a binder 209, which may be astencil paste, for example, that includes the metal particles 208. Thebinder 209 including the metal particles 208 is disposed atop a bottomsubstrate 202. In FIG. 6B, showing assembly 605B, a top substrate 200and an insert 204 are respectively disposed atop and around the binder209. The insert 204 includes a low melting point material that may be,for example, tin 206. The insert 204 includes a top surface S that hasan outer exposed portion and an inner portion disposed between surfacesof substrates 200 and 202. Thus, a top substrate 200 and a bottomsubstrate 202 are provided (i.e., block 900 of FIG. 9), and an insertholding tin 206 and a plurality of metal particles 208 (in an organicbinder) are positioned between the substrates (block 902).

FIG. 6C shows assembly 605C in which heat is applied to the assembly605C to a temperature below a melting point of the low melting pointmaterial as set forth in block 904 (i.e., below the melting point of tin206) and held at the temperature (block 906) such that the binder 209evaporates completely or substantially completely. For example, thebinder 209 may evaporate through an evaporation pathway in the directionof arrow E through one or more porous pathways and/or channels of theinsert 204. Evaporation of the binder 209 from the area of gap G of theinsert 204 may permit the avoidance of voids that may otherwise formwithin a resulting bonding layer 210. As a non-limiting example, thetemperature of the applied heat may be in the range of about 170° C. toabout 180° C. The temperature may be held for a period of time that maybe approximately 5 minutes.

In block 908, the assembly 605D is heated to a second temperature thatis approximately the melting point of tin 206. FIG. 6D shows assembly605D in which the melting point of tin is reached and the tin is meltedwhile the assembly 605D is held for a period of time (block 910). Forexample, tin 206 is melted and drawn into a gap G by capillary forces inthe direction of arrows A1 and B1 after the melting point temperature oftin 206 is reached. As an example and not a limitation, the temperaturereached in FIG. 6D may be in the range of about 230° C. to about 260° C.In embodiments, the period of time at which the temperature is held maybe in the range of about 5 minutes to about 10 minutes.

In block 912-914 of FIG. 9, with respect to the assembly 605E of FIG.6E, the temperature is heated and held at slightly higher than themelting point of the tin 206 until the tin 206 substantially fills thegap G. Tin 206 substantially fills in the gap G of FIG. 605D to arriveat the assembly 605E shown in FIG. 6E in which the metal particles 208are present and intermixed with the tin 206. The temperature may be heldat, for example, a temperature in the range of about 250° C. to about270° C. for a holding time of about 5 to about 10 minutes.

With respect to blocks 916-918, the temperature may then be increased toa higher temperature and held to cause a reaction between the lowmelting point material (such as tin 206, that may have a meltingtemperature in the range of about 230° C. to about 260° C.) and the highmelting point material (of the metal particles 208) to form a bond layer210 as shown in assembly 605F of FIG. 6F. This reaction causes theformation of the bond layer 210 by creation of one or more intermetallicbonds or alloys formed between the intermixed and heated tin 206 andmetal particles 208. In embodiments, the period of time at which thetemperature is held to create the bond layer 210 may be in the range ofabout 30 minutes to about 60 minutes. As a non-limiting example, thetemperature may be held in the range of about 290° C. to about 300° C.during this period of time. When formed, as by the methods describedherein, the bond layer 210 bonds the two substrates 200, 202 together.

While FIGS. 6A-6F depict an embodiment in which a gap G of the insert204D is shaped to hold an organic binder including the metal particles208 while the insert 204D initially holds the tin 206, the gap G mayinstead be shaped to hold a dry powder including the metal particles 208while the insert 204G is porous and shaped to initially hold the tin206, an organic solve, and a flux. For example, FIGS. 7A-7G depictanother alternative embodiment of a bonded substrate assembly 705Gformed through an alternative method, which may be, for example, themethod of FIG. 10. It should be understood, however, that other methodsdescribed herein, including those incorporating a temperature profile asshown graphically in FIG. 11, may be applied to assemblies 705A-705G asshown in respective FIGS. 7A-7G.

FIG. 7A shows an assembly 705A including an insert 204E that is disposedatop a bottom substrate 202 and that defines a gap G. In the illustratedembodiment, the insert 204E is porous and includes a low meltingtemperature material, such as tin 206. The tin 206 may be provided in apaste further including an organic solvent (i.e., having a viscosityless than the viscosity of an organic binder) and flux. In FIG. 7B,showing assembly 705B, dry metal particles 208 of a high melting pointmaterial are disposed in the gap G of the insert 204E. In FIG. 7C, a topsubstrate 200 is disposed atop the insert 204E, forming assembly 705C.The insert 204 includes a top surface S that has an outer exposedportion and an inner portion disposed between surfaces of substrates 200and 202. Thus, a top substrate 200 and a bottom substrate 202 areprovided (i.e., block 900 of FIG. 9), and an insert holding tin 206 anda plurality of metal particles 208 (as a dry powder, for example) arepositioned between the substrates (block 902).

As set forth in block 1004, FIG. 7D shows assembly 705D in which heat isapplied to the assembly 705D to a temperature below a melting point ofthe low melting point material (i.e., below the melting point of tin206) and held (block 1006) such that the paste material of the insert204E evaporates completely or substantially completely in, for example,a direction of arrows E. As a non-limiting example, the temperature ofthe applied heat may be in a range of about 150° C. to about 200° C. (asshown in the temperature profile graph of FIG. 8) or the range of about170° C. to about 180° C. The temperature may be held for a period oftime that may be approximately 5 minutes.

As set forth in blocks 1008-1010, the assembly 705E is then heated to asecond temperature above the melting point of tin (i.e., heated to andpassing the melting point of tin) and held at the temperature. FIG. 7Eshows assembly 705E in which the melting point of the tin is reached andthe tin is substantially melted. For example, tin 206 is melted anddrawn into the gap G housing the particles 208by capillary forces in thedirection of arrows A2 and B2 after the melting point temperature of tin206 is reached. As an example and not a limitation, the temperaturereached in FIG. 7E may be in a range of about 250° C. to about 350° C.(as set forth in block 1008 and as shown in the temperature profilegraph of FIG. 11). Alternatively, the temperature reached in FIG. 7E maybe in the range of about 230° C. to about 260° C. (i.e., as set forth inblock 808 of the example process of FIG. 8). In embodiments, the periodof time at which the temperature is held may be in a range of about 30minutes to about 120 minutes (as shown in the temperature profile graphof FIG. 11). In alternative embodiments, the period of time may be inthe range of about 5 minutes to about 10 minutes if, for example, thetemperature is to be incrementally increased (such as by a process setforth in FIG. 9) and further held prior to forming a final bond layer210 of FIG. 7G.

With respect to the assembly 705F of FIG. 7F and block 1010 of FIG. 10,for example, the temperature is held at slightly higher than the meltingpoint of the low melting point material (i.e., tin 206) until the tin206 substantially fills the insert gap G to arrive at the assembly 705Fin which the metal particles 208 are present and intermixed with the tin206. The temperature may be held at, for example, a temperature in therange of about 250° C. to about 270° C. for a holding time of about 5 toabout 10 minutes.

The temperature may further be increased to a higher temperature andheld to cause a reaction between the low melting point material (such astin 206, that may have a melting temperature in the range of about 230°C. to about 260° C.) and the high melting point material (of metalparticles 208) to form a bond layer 210 as shown in assembly 705G ofFIG. 7G. This reaction causes the formation of the bond layer 210 bycreation of one or more intermetallic bonds or alloys formed between theintermixed and heated tin 206 and metal particles 208. In embodiments,the period of time at which the temperature is held to create the bondlayer 210 may be in the range of about 30 minutes to about 60 minutes.As a non-limiting example, the temperature may be held in the range ofabout 250° C. to about 300° C. during this period of time. When formed,as by the methods described herein, the bond layer 210 bonds the twosubstrates 200, 202 together.

As an example and not a limitation, insert 204A-E of FIGS. 1A-7G isreusable. For example, any one of inserts 204A-E may be used with anadditional bonding assembly that is created via the example process setforth in FIGS. 8-11 and described herein. Additionally or alternatively,an array of multiple top and bottom substrates 200 and 202 may be usedwith selected ones of inserts 204A-E disposed therebetween to form orcreate a set of mass producible and multiple bonded assemblies (such asinserts 204A to create multiple bonded assemblies 105B) by the examplemethods described herein such as the one depicted in FIG. 1. Inembodiments, inserts 204 may be formed of multiple adjoining parts, suchas two half parts connected together, so to be laterally released fromdifferent sides of a pair of bonded substrates (for example, this allowsa lateral release of the insert 204 should the bond layer 210 formwithin slots 214 of the insert 204).

It should now be understood that embodiments described herein aredirected to methods for high temperature bonding of substrates todevelop a strengthened bonding or bond layer between two bonded twosubstrates. The bond layer is formed utilizing, in some embodiments, amulti-step process that incrementally heats and holds the substrates atincreasing temperatures. In some embodiments, paste that may be used toinitially releasably but adhesively join the two substrates together isevaporated after a first heating and holding step. In a second heatingand holding step, tin solder may freely coat metal particles without animpedance or restriction to this coating path that a present paste mightotherwise present. In other embodiments, capillary forces draws andconsumes tin solder as tin is melted into a bond layer formed betweentwo substrates being bonded in an example high temperature bondingprocess. The example methods described herein result in a strengthenedbond layer between two bonded substrates that may be used to bondsemiconductor devices in power electronics applications, for example.

It is noted that the terms “substantially” and “about” and“approximately” may be utilized herein to represent the inherent degreeof uncertainty that may be attributed to any quantitative comparison,value, measurement, or other representation. These terms are alsoutilized herein to represent the degree by which a quantitativerepresentation may vary from a stated reference without resulting in achange in the basic function of the subject matter at issue.

While particular embodiments have been illustrated and described herein,it should be understood that various other changes and modifications maybe made without departing from the spirit and scope of the claimedsubject matter. Moreover, although various aspects of the claimedsubject matter have been described herein, such aspects need not beutilized in combination. It is therefore intended that the appendedclaims cover all such changes and modifications that are within thescope of the claimed subject matter.

What is claimed is:
 1. A method for bonding of substrates, the methodcomprising: providing a top substrate and a bottom substrate;positioning an insert between the top and bottom substrates to form anassembly, wherein: the insert is positioned around at least a portion ofthe top substrate and comprises at least one of a porous material andone or more channels; the insert comprises a gap that is shaped to bedisposed between the top and bottom substrates and that is configured tohold a plurality of metal particles having a metal particle meltingtemperature, wherein at least one of the insert and the gap holds anamount of Sn having a Sn melting temperature; and the metal particlemelting temperature is greater than the Sn melting temperature; heatingthe assembly during a first heating to a first temperature that is belowthe Sn melting temperature; holding the assembly at the firsttemperature for a first period of time; heating the assembly during asecond heating to a second temperature that is approximately equal tothe Sn melting temperature; holding the assembly at a holdingtemperature for a second period of time such that the amount of Sn andthe amount of the plurality of metal particles form one or moreintermetallic bonds, wherein the holding temperature is equal to orgreater than the second temperature; and cooling the assembly to createa bonded assembly.
 2. The method of claim 1, further comprisingpositioning a paste between the top and bottom substrates, wherein: thepaste comprises an organic binder; the amount of the plurality of metalparticles is disposed in the paste; and heating the assembly during thefirst heating and holding the assembly at the first temperature for thefirst period of time further comprises vaporizing the paste.
 3. Themethod of claim 1, wherein the insert is porous, and the method furthercomprises positioning a powder containing the plurality of metalparticles in the gap of the insert, and disposing a paste comprising asolvent, flux, and the amount of Sn within one or more pores of theinsert.
 4. The method of claim 1, further comprising positioning apowder on surfaces of the top and bottom substrates such that the powderis disposed between the top and bottom substrates, wherein the amount ofthe plurality of metal particles is disposed in the powder.
 5. Themethod of claim 1, wherein: heating the assembly during the secondheating to the second temperature that is approximately equal to the Snmelting temperature comprises drawing Sn into a bond layer between thetop and bottom substrates via a capillary force such that Sn is drawninside and consumed into the bond layer; Sn wets the plurality of metalparticles; and the bond layer has a thickness in a range of about 10 μmto 250 μm after cooling.
 6. The method of claim 1, wherein the topsubstrate comprises a die made of Si or SiC.
 7. The method of claim 1,wherein the bottom substrate comprises direct bonded copper or directbonded aluminum.
 8. The method of claim 1, wherein: the insert comprisesgraphite or stainless steel and is circular; and the insert comprises athickness in a range of about 1 mm to 2 mm.
 9. The method of claim 1,wherein the at least an amount of Sn comprises a weight percent of 70%Sn and the amount of the plurality of metal particles comprises a weightpercent of 30% Ni.
 10. The method of claim 1, wherein each metalparticle comprises a material that is selected from at least one of agroup consisting of Ni, Cu, Ag, and Al.
 11. The method of claim 10,wherein the least an amount of Sn comprises a weight percent of 70% Snand the amount of the plurality of metal particles comprises a weightpercent of 30% of the material that is at least one of a group selectedfrom Ni, Cu, Ag, and Al.
 12. The method of claim 1, wherein each ofheating the assembly during the first heating and the second heatingcomprises undergoing one of a transient liquid phase heating or adiffusion soldering.
 13. The method of claim 1, wherein, after heatingthe assembly during a second heating to a second temperature that isapproximately equal to the Sn melting temperature, further comprising:holding the assembly at the second temperature for a first intermediaryperiod of time in a range of about 5 minutes to about 10 minutes;heating the assembly during a third heating to a third temperature thatis greater than the Sn melting temperature; holding the assembly at thethird temperature for a second intermediary period of time in a range ofabout 5 minutes to about 10 minutes; heating the assembly during afourth heating to the holding temperature that is greater than the thirdtemperature; and holding the assembly at the holding temperature for thesecond period of time that is in a range of about 30 minutes to about 40minutes.
 14. The method of claim 1, further comprising using an array oftop and bottom substrates with inserts disposed between each respectivetop and bottom substrate and applying the providing through coolingsteps to create a set of mass producible bonded assemblies.
 15. Themethod of claim 1, wherein: the first temperature is in a range of about170° C. to about 180° C.; and the first period of time is approximately5 minutes.
 16. The method of claim 1, wherein: the third temperature isin a range of about 250° C. to about 300° C.; and the second period oftime is in a range of about 30 minutes to 60 minutes.